The first three episodes of The Walking Dead's third season have been uniformly excellent and certainly not lacking in incident, but 'Killer Within' takes things to a whole new level. The demise of both Lori (Sarah Wayne Callies) and T-Dog (IronE Singleton) was truly unexpected; this isn't a season finale, this isn't even a mid-season finale. We always knew that anyone could go on this show, but it's now become abundantly clear that they can go at any time.

Following last week's Woodbury-set diversion, 'Killer Within' returns to the prison where the remaining inmates are begging Rick and company for mercy - it's a clear sign that Andrew Lincoln's scraggly former cop has fallen from grace when you find yourself backing the prisoners over our dictatorial 'hero'.

"Those two might actually have less blood on their hands than we do," points out the canny T-Dog - a character who's regrettably developed a set of vocal chords only to have them ripped out by the walkers.

Rick won't be talked down, but the darkness is alleviated somewhat as Hershel (Scott Wilson) gets back on his feet - even Rick and Lori share the briefest of smiles.

Any respite from the horror is tragically brief though, as a flood of zombies find their way into the prison grounds. The onslaught that follows is as tense, bloody and downright moving as The Walking Dead has ever been - T-Dog meets his maker in noble if bloody fashion, sacrificing himself to give Carol (Melissa McBride) a fighting chance, and then Lori, the queen of bad timing, goes into early labor...

The birth goes bad and, with Lori slipping away, it's left to Maggie (Lauren Cohan) to perform a primitive C-section. The baby survives, but Lori is gone - though given what happens in the Walking Dead comic, it's possible we haven't seen - or at least heard - the last of Sarah Wayne Callies yet.

We can certainly imagine Rick losing his mind going forward - not only is his wife gone, but he's also partially responsible for her death, having taken the decision to lock out crazed prisoner Andrew (Markice Moore) in the first place. For now though, let's take this chance to applaud Callies, Cohan, Lincoln and Chandler Riggs (Carl) on an emotionally devastating farewell.

Elsewhere this week, Michonne (Danai Gurira) begins to spot the first flaws in the Governor's apparent safe haven, remaining immune to the villain's charm. This writer picked up on a sinister, almost sexual undercurrent to his attempts to secure her silence - again, given what happens in the Walking Dead comic, it might not be entirely imagined. Then again, perhaps it's Andrea (Laurie Holden) who's being targeted - her flirtatious relationship with David Morrissey's maniac certainly has the potential to spiral horribly out of control...

Right now though, Andrea's considerably less cynical than Michonne about life in Woodbury - she even appears to be warming to the creepy Merle (Michael Rooker). In fairness, the blade-bearing brute is becoming more sympathetic with each passing week; there's an appealing amount of texture to both Rooker's performance and the way that Merle's now written - refreshing when you consider how easy it would be to portray a character like that as a one-dimensional racist redneck.

Merle's deal with the Governor - that if concrete information on Daryl's location is found, the two men will track him down together - also hints at the enticing inevitability of this season's two major story strands coming together - given how the Governor previously treated that military troupe, we imagine any meeting would be explosive.

Where could The Walking Dead go from here? We're not entirely sure, but following an episode as astoundingly brilliant as 'Killer Within', there's no way in hell we're not sticking around to find out.